Reland "Reland "[web tests] Add layout-test-results/ prefix to all artifact relative paths in json results"""

This reverts commit 1e90bd059625b28986a136473268e0049e2d1806.

In run_webdriver_tests.py, the output_dir variable is only created when the
--output-dir command line arguement is not used. If we try to pass
this value to the start_wptserver function when the output directory
command line argument is used, then an exception will be raised. This
CL will set the output_dir value to the command line value passed when
the --output-dir command line arguement is used

TBR=johnchen@chromium.org

Bug: chromium:1026494
Change-Id: Id943dcbc805bf0e82b1e3fb7a2d207e2885db7ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1965951
Commit-Queue: Rakib Hasan <rmhasan@google.com>
Reviewed-by: Robert Ma <robertma@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#724953}
19 files changed